Connection assembly for coaxial conductor



June 25, 1963 J. R. BIRD ETAL CONNECTION ASSEMBLY FOR COAXIAL CONDUCTOR Original Filed Feb. 4, 1953 5 M, V. wwww m s M m 5 Mm W50 A m AP JMB United States Patent 3,095,545 CONNECTION ASSEMBLY FOR COAXIAL CONDUCTOR James R. Bird, Chagrin Falls, and Harold E. Stevens,

Lyndhurst, Ohio, assignors to Bird Electronic Corporation, Cleveland, Ohio, a corporation of Ohio Original application Feb. 4, 1953, Ser. No. 335,118, now Patent No. 2,884,603, dated Apr. 28, 1959. Divided andthis application Mar. 11, 1959, Ser. No. 798,728

9 Claims. (Cl. 333-42) This invention relates to high frequency electrical devices and has particular reference to coaxial line components of the refiectionless termination type used either alone for attenuation or absorption of wave signal energy or in conjunction with suitable detector, sampling, pickup or voltage dividing devices as a measuring instrument or the like. Reference is made to co-pending parent application for United States patent, Serial No. 335,118, filed February 4, 1953, now Patent 2,884,603, dated April 28,

1959, of which this is a division.

The transmission, absorption and measuring of high frequency electrical energy is more diflicult in many respects than the corresponding treatment or handling of low frequency or direct current electrical energy because of capacitive and inductive effects, reflections and other phenomena. High frequency energy is satisfactorily transmitted on coaxial lines and it is known that a coaxial transmission line can be terminated substantially reflectionlessly by a device which incorporates coaxial inner and outer conductors, one of which is tapered and one of which is resistive, the tapered conductor desirably taking the form of a tubular metal horn surrounding a cylindridcal insulator of ceramic or like material on which is deposited or otherwise formed a film of carbon, or equivalent, the latter constituting the inner conductor of the line termination. In such arrangement the smaller end of the outer horn conductor makes direct electrical connection with resistive carbon film at one end of the inner conductor, thus completing the circuit.

Electrical energy fed into a line termination of the character mentioned is absorbed all along the resistive inner conductor, resulting in heating of the carbon film. At low energy levels the energy is satisfactorily dissipated into the atmosphere as by blowing air over the resistive film or by permitting air to circulate over the resistive fihn through the action of convection currents. It has also been proposed to employ a liquid dielectric coolant in the annular tapered dielectric space which separates the inner and outer conductors of the termination for forced flow or convective circulation of such coolant through the dielectric space either axially of the device or transversely thereof.

By reason of the geometry of the reflectionless device, requiring close proximity of the inner and outer conductors at one end of the device and wide separation thereof at the other end, and the need for substantially uniform heat absorption all along the length of the resistive conduct-or, servicing, checking and repairing such devices present numerous problems. The present invention is therefore concerned primarily with and has for its principal object the provision of a new apparatus for reflectionlessly terminating a coaxial transmission line which incorporates novel manufacturing, assembling and servicing advantages. More particularly, the invention is concerned with improvements in the mechanical and electrical connection between the small diameter end of the horn shaped outer conductor and the thin film inner conductor of a reflectionless coaxial line termination.

Another object is to provide such a connection which provides for relative axial shifting of the outer conductor relative to the inner conductor while maintaining both a 3,095,545 Patented June 25, 1963 2 strong mechanical grip of the outer conductor on the inner conductor and good electrical contact between the conductors. According to this aspect of the invention the outer conductor is made yieldable by slitting and is embraced by resilient means which causes it yieldingly to clamp the inner conductor which it surrounds.

Another and more specialized objective is to provide a termination of the type referred to having an inner cylindrical conductor axially withdrawable from and replaceable in a tapered horn outer conductor and in which the mechanical and electrical connection between the conductors at the small end of the horn incorporates improved means by which the horn holds the inner conductor in a yielding grip.

A further object is to provide such a termination having at the small end of the horn a short cylindrical portion defining clearance for circulation of fluid coolant between the conductors, having a capacitance stub within the inner conductor, and having the stub secured to and wholly supported by the inner conductor for removal and replacement therewith.

Other objects relate to certain novel combinations of parts and features of construction of advantage in axial insertion and withdrawal of the inner conductor structure. Such and still other objectives and advantages are apparent in the following detailed description of a preferred en1- bodiment representing the best known mode of practising the invention. The description is made in connection with the accompanying drawings forming a part of the specification.

In the drawings:

FIGS. 1 and 2 are fragmentary sectional views taken longitudinally through the upper and lower portions respectively of a reflectionless termination component and casing of a high frequency electrical meter instrument which embodies the principles of the present invention;

FIG. 3 is an axial detail view of the small diameter end of the reflectionless termination showing spring clips for maintaining resilient contact between the resistive center conductor and the tapered metal outer conductor at the small end of the termination; and

FIG. 4 is an enlarged sectional detail, diagrammatic in character, showing the terminal contact band of thin brass soldered about the copper plated end of the resistive inner conductor at the lower or small end of the termination.

The high frequency electrical device illustrative of the present invention comprises a reflectionless termination T received in an elongated casing C, the latter preferably taking the form of a cylindrical metal tube 1 of brass or steel, although other cross sectional shapes can be used. The top and bottom ends of the casing are closed by circular metal plates or discs 10 and 11, respectively, the latter being permanently secured in place as by peripheral welding. The top disc 10 is held in place by clamping ring 14 with an interposed rubber annulus.

In operating the device liquid dielectric coolant is flowed upwardly into the bottom of the casing C through tube 28 which extends into the bottom of the casing through a circular opening '30 in the bottom closure 11.

The termination T has an inner resistive conductor in the form of a carbon film 40, or equivalent, deposited or otherwise formed on the outer cylindrical surface of a ceramic tube 41. Surrounding the inner resistive conductor in coaxial relation thereto is an outer tapered condoctor 42 which may take the form of a metal horn spun or stamped from brass or similar thin metal. The horn is shaped in accordance with known principles to provide a characteristic impedance at any point or section which is substantially equivalent to the resistance of that portion of the resistive inner conductor 40 which lies between the point or section of reference and the small end of the termination where the inner and outer conductors are 'oined.

1 At its upper or large diameter end tapered outer conductor 42 is formed with an integral cylindrical extension or connector section received within a socket opening through the bottom end of a body member 45 which is of circular section and may be a turning of brass. The conductive parts of the device are machined from brass stock, except as otherwise indicated or implied. The circular body 45 has an external cylindrical surf-ace 46 that has a freely sliding fit in cylindrical reinforcing band 47 press fitted into the body tube of the casing. The bottom end of a cylindrical relatively thin Walled metal connector tube is received in shallow socket in upper end of the body 45. This connector tube extends upwardly therefrom through the upper portion of the easing in coaxial relation and through a central opening 51 in the top closure -19. The connector tube 50, of brass or copper, is secured in the shallow socket 49 as by soldering or brazing and additionally by a plurality of radial set screws threaded through the termination body member 45 and the wall of the connector tube so as to bear against and become embedded in a plastic sealing ring 127 to be later described.

A ring 54 is formed on or welded against the underside of the top closure about th center opening that receives the connector tube 5 1?. Radial set screws 56 are threaded through the ring 54 and are received through openings in the connector tube so as to bear radially against and become interlockingly embedded in an annular block insulator 6!) of suitable dielectric plastic material. Rubber O-ring 59 is an annular groove formed in the surface of the insulator block 6i engages the internal surface of the connector tube 59 in the provision of a fluid tight seal. Slots 61 in the connector tube 50 place the interior chamber of the connector tube in com munication with an upper annular casing chamber above the circular body 4-5 and in communication with a suit able heat exchanger (not shown) through connecting tube 22.

The lower end of the ceramic tube 41 is embraced by the small end of the outer or horn conductor 42; the upper end of the ceramic tube is connected to a center conductor connector '64 within and coaxial to the connector tube 50. The center conductor 64 is 'formed at its lower end with an internal socket 65 that receives the upper end of the ceramic tube 41, the latter being provided with a terminal band 67 which extends onto and makes annular electrical connection with the end portion of the deposited carbon film coating.

An inner terminal 73 comprises a turning of brass formed at its upper end with a cylindrical cup 74 which is adapted to receive a connector plug of a coaxial transmission line. This terminal is formed with a reduced diameter shank portion that extends axially through center opening or bore 76 in the insulator 69.

The terminal elements 73 is formed with axial slots 87 which provide a number of tines or fingers. At its end opposite the terminal cup 74 the element 73 is formed with a reduced diameter end portion receivable in a matching socket of an intermediate element 91 that has a telescoping connect-ion with the center conductor connector 64. Embracement of the lower end of the element 73 by the walls of the element 91 prevents expansion of such lower end of the slotted shank under radial pressure of an internal expander (not shown) so that the expansion of the terminal element takes the form of an outward or radial bowing of each of the tines or fingers. Rubber O-ring 93 in an annular recessed groove in the wall of the bore 76 in the plastic insulator engages the cylindrical external surface of the intermediate element 9 to seal the bore 76.

At the small end of the termination the tapered outer conductor 42 includes a relatively short integral cylindrical end section or portion 95 to provide an annular radially shallow clearance space of uniform circular section surrounding the resistive film 40 of the center conductor. This annular clearance is more effective for the flow of liquid coolant than would be the tapered clearance space resulting from tangential contact between the curved horn and the cylindrical film resistor, as called for by theoretical considerations governing the design of reflectionless terminations for coaxial lines. To compensate for the departure from such theoretical considerations resulting from use of the cylindrical end section 95, a capacitance compensating inductive metal tube 102 is disposed inside the end of the resistive conductor tube 41 and electrically connected to the small end of the conductor born 42. The present device incorporates certain features of construction and combinations of parts that are improvements on a similar and earlier cylindrical end section compensating tube arrangement described in application Serial No. 72,782, now United States Patent 2,752,572, dated June 26, 1956 for Liquid-Cooled Load for a Coaxial Transmission Line.

Contact is made between the horn conductor 42 and the resistive inner conductor 40 by a reduced diameter cylindrical end d7 which is integral with and extends as an axial continuation or extension of the cylindrical portion 95. The small cylindrical end portion or section 95 of the horn conductor 42 is formed with apertures such as axially elongated slots $8 for admitting liquid coolant to the dielectric space between the termination conductors, as will be described later. These slots extend the full length of the cylindrical portion 95 and are spaced uniformly about the circumference of such cylindrical portion. At least one of the slots Q8 is continuous through the reduced diameter cylindrical end portion 97, as indicated at 123 (FIGS. 3 and 4), to permit expansion and contraction of the latter in yieldingly gripping the end of the inner conductor. Electrical connection may be made directly between the carbon film coating 40, comprising the inner conductor, and the cylindrically curved inner surface of the extreme end or terminal portion 7 of the metal horn conductor 42. Suitable yielding clamping means such as a plurality of axially spaced parallel split spring rings 95 is provided to contract resiliently about the cylindrical end portion 97. The cylindrical terminal end of the horn conductor is thus held or compressed conformatively against the inner conductor for good electrical contact with the latter.

As a preferred arrangement for connecting the compensating tube 102 to the horn 42, and one which obtains certain advantages in the way of protection for the carbon film 40 by preventing scraping and burning thereof, the present device employs a cylindrical terminal element or band 1%. This conductive metal band may take the form of a strip of thin sheet metal such as brass, having a thickness, say, of about .005 inch and commonly known as shim stock. This band is wrapped about the end portion of the carbon film 46 on the ceramic tube 41 and permanently secured in place by an underlying film 94 (FIG. 4) of solder. The solder bonds to a suitable conductive film or coating of metal such as copper electroplated or otherwise formed as a cylindrical band on the end portion of the carbon film. The brass terminal band 1% extends axially beyond the lower end of the ceramic tube 41 and receives or embraces a ring 101 of brass or othermetal which is located and abutted protectively against the lower end of the ceramic tube. The ring 101 embraces the lower end of the conductive capacitance compensating tube 102 of brass or other similar metal which is received or telescoped within the lower end of the ceramic tube 41. The ring M21 is electrically and mechanically connected by the solder film 94 to the metal terminal element or band 100 and by solder film 113 to the tube 102. These solder joints are made, if desired, at the same time the terminal band 100 is soldered to the copper plating 105 on the resistive film conductor 40.

The metal capacitance compensating tube 102 is of such length that its upper or innermost end 103 is disposed approximately at the radial plane defined by the juncture indicated at 109 (FIG. 2) between the cylindrical end portion 95 and the tapered portion of the outer horn conductor 42. Capacitance between the conductive metal sleeve 102 and the film conductor 40 compensates for loss in capacitive coupling between the tapered horn conductor 42 and the resistive film conductor 40 resulting from using the cylindrical portion 95 at the small end of the horn conductor instead of continuing the design curvature of the tapered horn into tangential contact with the cylindrical surface of the resistive film conductor as called for by the theoretical considerations previously referred to which govern the design of reflectionless terminations.

The termination T is aligned axially with the entrant tube 28 through which liquiddielectric coolant is introduced into the casing C. Funnel 106 embraces both the end of the termination and inlet end 107 of the entrant tube 28. The funnel has a stem portion 108 received about the small end of the horn conductor 42 and slots 98 and 116 in the latter admit liquid dielectric coolant into annular space 96 between the horn conductor and the inner resistive film conductor 40.

At its lower end the funnel 106 is formed with a bell end 111 which surrounds the end 107 of the entrant tube 28. i

The liquid dielectric coolant is desirably maintained in a path closely surrounding the resistive inner film conductor 40 as by a tubular guide 118 of glass, plastic or other suitable dielectric material and which surrounds the inner conductor in coaxial relation. At its lower end adjacent the small end of the termination T the glass guide 118 is telescoped into a collar or thimble 120' of plastic dielectric material formed with a tapered external surface which fits the internal tapered contour of the outer conductor 42. .An opening or passage 122 through the wall of the thimble 120 permits pressure equalizing flow between the chamber portions separated by the tubular guide 118. At its other end the glass guide 118 has flared end portion which is received in an annular rabbet of plastic locating and sealing ring 127.

To insure a small fluid flow through the chamber that surrounds the guide 118 the sealing ring 127 is formed with an axial passage 128 whichthus connects the chamber portion with the space between the center conductor connector 64 and the connector tube 50. The passage 128 in the plastic ring 127 and the passage 122 in the plastic thimble 120 are respectively located substantially at the extreme upper and extreme lower ends of the chamber portion 124 to result in a limited flow of liquid dielectric coolant through such apertures or openings into and out of such chamber portion.

The maximum diameters and cross sectional areas of the guide tube 118 and of the plastic sealing rings 120 and 127 are each less than the diameter and cross sectional area ofthe interior opening or passage of the connector tube '50 so that in assembling the guide tube 118, the thimble 120 and the sealing ring 127 into the outer conductor structure of the termination, the guide tube may be inserted axially into place through the open terminal end of the connector tube 50 prior to insertion therein of the annular insulator block 60 and the inner conductor assembly.

In commercial production of the device the circular body 45, the connector tube 50' and the tapered horn conductor 42 are conveniently assembled together i in the telescoped relation shown while outside of the casing body tube 1. The guide tube 118 is also moved into place prior to assembly into the line of the inner conductor components and while the outer conductor components as a television transmitter.

are outside the casing tube 1. The annular insulator block 60 is assembled about the inner terminal element 73 by sliding it axially over the inner conductor tube 41 to the desired location about the shank of the inner terminal element 73, the block being located axially in such assembly by a plastic dielectric washer 130 interposed between the block and the shoulder provided by the relatively large diameter head end of the inner terminal element 73. The center conductor structure of the termination, comprising the ceramic tube 41 with its terminal band 100, the capacitance compensating tube 102, the connector 64, the terminal 73, and the dielectric insulating block 60, is then assembled with the outer conductor structure.

Prior to the insertion of the termination T or the outer conductor 42 thereof into the casing C the funnelel-ement 106 is assembled onto the small end of the outer conductor by telescoping such element into place and soldering or brazing the end of the stem portion 108 to the external surface of the outer conductor 42. The spring clamping rings 99 areassembled about the axially split cylindrical end 97 of the outer conductor either before or after the inner conductor structure has been moved into place. The clamping force of the split conductor end 97 about the terminal band on the inner conductor is insuflicient, however, to prevent axial adjusting movement of the inner conductor structure relative to the outer conductor structure. Thus in telescopically assembling the tubular guide 118 and the inner conductor structure with the outer conductor structure prior to the insertion of the latter into the casing body 1, the parts may be easily and accurately placed as by means of suitable assembly igs.

The permissible axial slipping or shifting of one conductor structure relative to the other, mentioned above, is a desirable feature not only because of the accommodation of manufacturing variations which permits greater tolerances and the ability of the device to expand and contract differently upon temperature changes encountered in practice but because of further advantages incident to service and repair in the field. It may occur, for example, that one of the inner conductor components such as the resistive film 40 or the ceramic tube '41 becomes damaged in use or the instrument may appear to be liquid dielectric coolant from the casing C. The interlock and grip between the insulator block 60 and the terminal element 73 is first released. The inner conductor assembly, no longer held against relative turning or relative axial movement by the insulating block 60, can be withdrawn from or returned into the outer conductor assembly. The bore 76 in the insulating block 60 is, of course, slightly larger in diameter than the withdrawn parts so that these parts may be withdrawn and inserted axially through the insulating block 60 while the latter remains permanently assembled and secured in place. The brass terminal band 100 bears the frictional load imposed by the split clamping end 97 of the horn conductor which, under the resilient squeezing action of the spring rings 99 and by reason of the inherent resilience of the brass or other metal stock from which the horn is formed, must be slightly distended by the terminal band 100 to admit the latter in assembly.

The termination T can be used to absorb the energy from a high frequency electrical signal generator such It may be combined with a suitable signal responsive system including a detector from the transmitter in accordance with known principles. In such an instrument a pickup device or detector D, like the unit assembly D described in application Serial No. 99,474 filed June 16, 1949, now Patent 2,636,120 dated April 21, 1953, is combined with the termination T so that the parts function together in accordance with the principles described in the application referred to.

The unit D includes a chambered body member 76 of brass or similar metal formed with a cylindrical pilot extension 133. A flat mounting plate 135 is formed with a central aperture which receives the cylindrical extension 133 of the body and is abutted against a shoulder of the latter, being soldered to the body and to the cylindrical extension 133.

A circular opening 137 to receive the pilot extension 133 is formed through the wall of the casing 1, through the reinforcing band or ring 47, and through the telescoped body 45 and connector section 44- of the termination outer conductor.

The present invention thus provides useful improvements in specialized apparatus for converting high frequency electrical energy into heat and dissipating such energy.

In accordance with the patent statutes the principles of the present invention may be utilized in various ways, numerous modifications and alterations being contemplated, substitution of parts and changes in construction being resorted to as desired, it being understood that the embodiment shown in the drawings and described above is given merely for purposes of explanation and illustration without intending to limit the scope of the claims to the specific details disclosed.

What we claim and desire to secure by Letters Patent of the United States is:

*1. In a high frequency electrical device of the type comprising an inner resistive conductor and a tubular outer conductor arranged as a coaxial line and in which the inner conductor comprises an insulator tube carrying a resistive film, improved means for connecting the inner and outer conductors at one end of the line, said connecting means comprising a conductive element received within and a conductive band embracing and carried by the tube at said end of the line, the conductive band being electrically connected to the resistive film and constituting a terminal of the inner conductor at said one end, means electrically connecting the element to the band, said element being fast to and wholly supported by the tube independently of the outer conductor for movement of the tube and the element as a unit relative to said outer conductor, the outer conductor having a terminal portion embracing the terminal band in a slip joint for relative axial sliding movement to accommodate longitudinal shifting of one conductor relative to the other, said slip joint providing electrical connection between the outer conductor and both the resistive film and the conductive element in different shifted positions, and the conductive element and the resistive film comprising a capacitive coupling between the conductors at said one end of the line.

2. In a device as in claim 1 the connecting means between the internal conductiveeiement and the terminal band comprising a metal ring located beyond the end of the insulator tube.

3. In a device as in claim 1 the terminal band comprising an inside metal film surrounding the resistive film and in direct electrical contact therewith and a cylindrical metal element embracing and soldered to the metal film.

4. In a device as in claim 3 the metal film and the metal element having corresponding ends axially spaced with an annular surface portion of the metal film exposed between such ends.

5. A high frequency electrical resistive device comprising an inner resistive conductor and a tubular outer conductor arranged as a coaxial line, the conductors being separated by an annular dielectric space and converging into electrical contact with one another at one end of the device, the inner conductor comprising an insulator tube having a resistive film of cylindrical shape, a conductive terminal element carried by the tube and electrically connected to the resistive film at said one end of the device; conductive means carried by the tube, said conductive means paralleling the resistive film and being electrically connected to the terminal element, the outer conductor having a terminal portion yieldingly embracing the terminal element of the inner conductor in a slip joint for axial movement of the outer conductor relative to the tube and the conductive means to accommodate longitudinal shifting of one conductor relative to the other, said slip joint providing electrical connection between the outer conductor and the resistive film in different relatively shifted positions, said conductive means providing capacitive coupling between the resistive film and the outer conductor when the terminal portion of the latter is in electrical contact with the terminal element, and the conductive means being fixed to the tube for movement with the inner conductor and relative to the outer conductor upon such sliding movement.

6. In a highvfrequency electrical device comprising a resistive conductor and a tubular conductor arranged as a coaxial line, the conductors being separated by an annular dielectric space and converging into electrical contact with one another, the resistive conductor comprising a cylindrical tube insulator having a resistive film, a conductive terminal element carried by the insulator and electrically connected to the film, the tubular conductor having a terminal portion yieldingly engaging the terminal element at said contact in a slip joint permitting relative axial movement of the conductors while said contact is maintained, conductive means comprising a cylindrical element electrically connected to the terminal element, said conductive means being fixed to and carried by the resistive conductor for movement therewith relative to the tubular conductor in said relative move ment, one of said conductive means and terminal elements being disposed inside and the other outside the insulator tube, a conductive ring disposed against one end of the insulator tube and mechanically and electrically connected to both said conductive means and terminal elements, said conductive means being connected electrically to the tubular conductor through the terminal element and said slip joint, and said conductive means being disposed in spaced relation to a portion of said film adjacent said contact, said portion of the film being disposed between the tubular conductor and the conductive means with the conductive means comprising one element and the film comprising another element of a capacitor coupling the tubular conductor to the film.

7. In a high frequency device as claimed in claim 6, the terminal element comprising a cylindrical metal band having a diameter different than that of the cylindrical resistive film element.

8. In a high frequency device as claimed in claim 6, the terminal element comprising a cylindrical metal band projecting axially beyond said end of the insulator tube and in overlying relation to the conductive ring.

9. In a high frequency electrical device comprising a resistive conductor and a tubular conductor arranged as a coaxial line, the conductors being separated by an annular dielectric space and converging into electrical contact with one another, the resistive conductor comprising an insulator tube having a cylindrically shaped resistive film, the tubular conductor having a terminal portion in engagement with and adapted for axial sliding in assembly relative to the resistive conductor at said contact, and conductive means disposed within and wholly supported by the insulator tube, a portion of the resistive film adjacent the contact being sandwiched between and disposed in spaced relation to the tubular conductor and the conductive means, means providing an axially slidable electrical connection between the conductive means ,and the terminal portion of the tubular conductor to upon disassembly and to effect reconnection thereof upon reassembly.

References Cited in the file of this patent UNITED STATES PATENTS 2,031,296 Alsaker Feb. 18, 1936 2,331,255 West Oct. 5, 1943 2,552,707 Bird et a1. May 15, 1951 2,752,572 Bird et a1. June 26, 1956 2,884,603 Bird et a1. Apr. 28, 1959 3,054,074 Bird et al. Sept. 11, 1962 

1. IN A HIGH FREQUENCY ELECTRICAL DEVICE OF THE TYPE COMPRISING AN INNER RESISTIVE CONDUCTOR AND A TUBULAR OUTER CONDUCTOR ARRANGED AS A COAXIAL LINE AND IN WHICH THE INNER CONDUCTOR COMPRISES AN INSULATOR TUBE CARRYING A RESISTIVE FILM, IMPROVED MEANS FOR CONNECTING THE INNER AND OUTER CONDUCTORS AT ONE END OF THE LINE, SAID CONNECTING MEANS COMPRISING A CONDUCTIVE ELEMENT RECEIVED WITHIN AND A CONDUCTIVE BAND EMBRACING AND CARRIED BY THE TUBE AT SAID END OF THE LINE, THE CONDUCTIVE BAND BEING ELECTRICALLY CONNECTED TO THE RESISTIVE FILM AND CONSTITUTING A TERMINAL OF THE INNER CONDUCTOR AT SAID ONE END, MEANS ELECTRICALLY CONNECTING THE ELEMENT TO THE BAND, SAID ELEMENT BEING FAST TO AND WHOLLY SUPPORTED BY THE TUBE INDEPENDENTLY OF THE OUTER CONDUCTOR FOR MOVEMENT OF THE TUBE AND THE ELEMENT AS A UNIT RELATIVE TO SAID OUTER CONDUCTOR, THE OUTER CONDUCTOR HAVING A TERMINAL PORTION EMBRACING THE TERMINAL BAND IN A SLIP JOINT FOR RELATIVE AXIAL SLIDING MOVEMENT TO ACCOMMODATE LONGITUDINAL SHIFTING OF ONE CONDUCTOR RELATIVE TO THE OTHER, SAID SLIP JOINT PROVIDING ELECTRICAL CONNECTION BETWEEN THE OUTER CONDUCTOR AND BOTH THE RESISTIVE FILM AND THE CONDUCTIVE ELEMENT IN DIFFERENT SHIFTED POSITIONS, AND THE CONDUCTIVE ELEMENT AND THE RESISTIVE FILM COMPRISING A CAPACITIVE COUPLING BETWEEN THE CONDUCTORS AT SAID ONE END OF THE LINE. 